"I'm not nervous, I'm only tense!"
Translation:Saya tidak gugup, saya hanya tegang!
Halo teman! The simple difference here is that "gugup" is an adjective, so it cannot fit well with "bukan". Still, there would be some variations that can use "bukan". I think that replacing "gugup" with "orang yang gugup" (not necessarily doing the same with "tegang"), or with "penggugup" (noun, "a person who easily becomes nervous"), this sentence has a different construction, but a close meaning.
Saya bukan orang yang gugup, saya hanya tegang!
Saya bukan penggugup, saya hanya tenang!
I think these sentences translate as "I am not a nervous person, I am only tense!"
Wiktionary bahasa Indonesia
I hope it helps.
Selamat belajar! :)